What actions earn credits?
Credits are used to purchase things like additional levels (that I've found so far). You can get more through microtransactions though I'd rather avoid this option:
What in game actions allow you to earn credits?
Completing a quest from the Emperor.
Completing a quest from Vader.
Taking a bitizen to their requested floor in the elevator.
Stocking an item in a shop (Food, Retail, Recreation, Service) will lead to credits as it will then be sold.
Rent being payed by the occupants of a residential level.
Taking a Big Spender to a shop with 1 or more products.